Most locking systems currently in use in hotels, motels, and similar applications have pin-tumbler locks fitted with corresponding keys, and, one or more levels of mastering are used. As a practical matter, it is impossible to prevent guests from taking or duplicating individual room keys. Compromising of master keys is also relatively common, since the background of hotel employees cannot always be thoroughly investigated.
Other systems which have been proposed, or used, replace the key with a card using optical, magnetic or other means of encoding. Such systems suffer from complete failure when power is lost, thus preventing a guest from entering his own room under such circumstances.
In U.S. Pat. No. 3,763,676 and U.S. Pat. No. 3,842,629, a system is proposed which overcomes this difficulty, but the construction requires power and signal wiring to each individual door. In the case of existing buildings, building codes requiring complicated installation of power lines make installation of such systems economically unsound. Further, with a failure of power, such systems are also inoperative.